In this respect, the present invention provides improvements to the means described in documents U.S. Pat. No. 5,523,739.
Metal detectors and transponder detectors have already given rise to abundant literature.
It is known that most metal detectors comprise transmitter coils, receiver coils, and an electronic processor circuit adapted to detect variations in the signals received by the receiving coils relative to a reference value.
Still more precisely, present-day metal detector systems generally satisfy the following criteria.
Firstly, the coils, both the transmitter coils and the receiver coils are of the multipolar type, i.e. they are in an 8-shape or the equivalent, so as to present alternating poles of opposite sign so as to cancel out or minimize interfering signals, e.g. as induced by metal masses or magnetic field sources placed close to but outside the detection space as defined by the transmitter and receiver coils.
Secondly, there are preferably a multiplicity of transmitter coils configured to generate multidirectional magnetic fields so as to guarantee coupling between the magnetic field and a metal mass passing between the transmitter and receiver coils, whatever the path followed by the mass and/or whatever its orientation. Similarly, there are preferably multiple receiver coils configured to detect the influence of a metal mass, regardless of the path it follows and/or regardless of its orientation.
Finally, and thirdly, the intensity of the transmitted magnetic fields must be controlled accurately and must remain below acceptable thresholds in order to avoid disturbing the state and/or the operation of certain authorized apparatuses that may pass between the transmitter and receiver coils, for example heart pacemakers.
Document U.S. Pat. No. 5,523,739 describes a system of the type shown diagrammatically in accompanying FIG. 1 which system combines a metal detector and a transponder detector for the purpose of enabling authorized people carrying a transponder to pass through the access even if they are carrying a weapon, without it being necessary for such people to draw attention to themselves by any detectable operation, such as keying a special number on a keypad or subjecting themselves to biometric reading, in particular of a fingerprint. Such detectable operations can be very harmful to the security of such people who are thus identified as being unusual.
More precisely, and as described in document U.S. Pat. No. 5,523,739, a system comprising:
transmitter coils 1 suitable for generating magnetic fields at audiofrequencies "foi"; PA1 associated receiver coils 2; PA1 transponders 3 each having an on-board electrical power supply, at least one coil closed by a high impedance circuit for the signals at the audiofrequency "foi" so that the coil is sensitive to such signals at the audiofrequency "foi" while nevertheless picking up little energy at said frequency and consequently not disturbing metal detection, and means for generating an identifiable signature, e.g. a determined code by modulating emission on the coil in response to detecting the audiofrequency "foi"; and PA1 processor circuits 4 suitable firstly to detect a weapon passing between the transmitter and receiver coils 1 and 2 by analyzing variations in a signal associated with the audiofrequency "foi" received by the receiver coils 2, and secondly for detecting the identifiable signature from the transponder 3. PA1 transmitter coils; PA1 associated receiver coils; PA1 transponders suitable for generating identifiable signatures; and PA1 processor circuits adapted firstly to detect a weapon passing between the transmitter and receiver coils by analyzing variations in the signal received by the receiver coil, and secondly to detect the identifiable signature of a transponder on the basis of the signal received by the receiver coils; PA1 the transmitter coils are adapted to generate respective frequencies, firstly for exciting and detecting any weapons passing between the transmitter and receiver coils, and secondly for activating transponders; PA1 the transponders have means suitable for powering their internal circuits electrically from the signal received at the "activation" frequency; PA1 each weapon excitation and detection frequency is very different from each frequency used for activating transponders and for transponder signatures; and PA1 the signals used for detecting transponders are taken from the outputs of receiver coils that are also used for detecting weapons.
The means proposed in document U.S. Pat. No. 5,523,739 thus make it possible to combine and integrate neatly a metal detector and a transponder detector in spite of apparent incompatibility between detectors of those two types, particularly as to the configuration of the detection coils used, the frequencies involved, and the intensities required, as explained in the preamble of document U.S. Pat. No. 5,523,739.
Nevertheless, the means described in document U.S. Pat. No. 5,523,739 have yet to be developed industrially on the large scale expected from the marked advantages that such means offer compared with the state of the art.
That would appear to be due in particular to the fact that if the power supply on the transponder fails, the system ceases to operate, since it deems any person carrying a weapon to be an unauthorized person even if that person is authorized but is carrying a defective transponder, without there being any reason why the person should be aware that the transponder is defective and understand why access is being refused exceptionally.
Another problem is the cost of a transponder with an in-built battery, and this factor limits commercialization to applications where large numbers of transponders are required. Another negative factor of battery transponders is their limited lifetime which makes the apparatus unusable after being active for a certain length of time.
The problems encountered in this way have not yet been solved in satisfactory manner, since any modification of the system relative to the means described in document U.S. Pat No. 5,523,739 fails because of the incompatibilities of principle that exist between metal detectors and hands-free transponder detectors, as recalled above.